A particle-detecting telescope sunk more than a mile beneath the sea off the coast of France has tested a core principle of Einstein's relativity and found no cracks.
The detector, called Oscillation Research with Cosmics in the Abyss (ORCA), detects elusive particles called neutrinos. As a doctoral student, Alba Domi rode onto the Mediterranean aboard one of two boats that helped build the telescope, lowering some of its first detection units to the seafloor about 1.5 miles (2.4 kilometers) below the waves off Toulon, France.
Now, data from those first detection units have been used to test one of the founding rules of Einstein’s relativity: the idea that the laws of nature look the same no matter which way you face or how fast you move.
The team found no sign that this rule, called Lorentz invariance, is broken. But even with a detector only a fraction of its planned size, they set limits that compete with those of far larger and longer-running experiments.
The results were published Wednesday (Nov. 4) in the Journal of Cosmology and Astroparticle Physics.
A crack in space-time?
Physics rests on two successful theories: general relativity, conceived by Albert Einstein more than 100 years ago, and the Standard Model of subatomic physics.
"General relativity describes the universe well on a cosmic scale, and the Standard Model of particle physics describes the microscopic scale; really, the single particles, how they behave," said study co-author Lukas Hennig, a doctoral student in advanced physics at Erlangen-Nürnberg.
Both theories assume Lorentz invariance to be true. But in extreme settings, such as the first moments after the Big Bang, the two theories don't mesh. That has fueled a decades-long hunt for a theory of quantum gravity that unites them. Some candidates, including versions of string theory and loop quantum gravity, suggest that space-time might have a hidden "grain" at the tiniest scales — and that Lorentz invariance, "one of the most basic assumptions we make about our universe, might be violated," Hennig said.
Any such violation would be extremely small. To spot it, physicists would need a probe that could travel long distances and barely interact with anything along the way. Enter the neutrino.
Shape-shifting ghosts
Neutrinos are nearly massless particles that pass through Earth every second and carry zero charge. They barely interact with anything and come in three types: electron, muon and tau neutrinos. As neutrinos travel, they morph from one type into another in a predictable pattern that physicists call neutrino oscillation.
An optical module attached to the Dyneema ropes of the detection unit. (Image credit: KM3NeT Collaboration)
If Lorentz invariance were broken, that oscillation would change, with the effect growing more pronounced the farther the neutrinos traveled. So the team focused on neutrinos created when cosmic rays slam into the atmosphere elsewhere on the globe before traveling up through the planet to the detector. Some of the tested neutrinos crossed the entire 7,900-mile (12,700 km) width of Earth.
ORCA is part of the KM3NeT neutrino telescope. Each of its detection units is anchored to the seabed and holds 18 glass spheres packed with 31 light sensors apiece. When a neutrino collides with a particle in the water, it produces a faint flash of blue light that the sensors catch. The many sensors in each sphere help the team pin down which direction the light came from, Domi said.
Earth itself serves as a filter. The researchers selected only particles traveling upward through the planet, and they were able to weed out the flood of other particles from the atmosphere that would otherwise swamp the signal. "Only the neutrinos can get through," Hennig said.
That setup differs from that of IceCube, the Nobel Prize-winning detector buried in Antarctic ice. "Ice scatters the light more than water, but on the other hand, water absorbs the light more than ice," Domi said, which changes how each team reconstructs a neutrino's energy and path.
Small but mighty
The researchers used 5,828 neutrinos collected over about 17 months between 2020 and 2021. They found that the oscillations matched standard physics. That means if Lorentz violation exists, it must be weaker than the limits the team set.
Those limits matter for theorists. If a model "predicts a Lorentz invariance violation [stronger than the new limits], the data rule it out. In that case, you can be pretty sure that this model is not correct," Hennig said.
The team also set the first direct limits on a kind of violation that would treat muon neutrinos and tau neutrinos differently.
The detector's limits on several key measures are competitive with those from Japan's Super-Kamiokande, which used 12 years of data, and from IceCube, which used two years of data from its full array.
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"I'm surprised that we have competitive results with only six detection units," Hennig said. Today, 44 units are in the water, and the collaboration plans to activate 108 by 2030.
The best tests may still lie ahead. Because Lorentz-violating effects grow with distance and energy, neutrinos from distant cosmic sources are "actually the best candidates to do this kind of research," Domi said. The team plans to repeat the search using ORCA's sister detector, ARCA (Astroparticle Research with Cosmics in the Abyss), which sits off the coast of Sicily and is designed to catch those high-energy cosmic travelers.
If space-time does have a grain, those particles may be among the first to feel it.
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